The present invention relates to a novel method for applying a thermoplastic coating material to an article of manufacture. Most particularly, the invention relates to a method for evenly and firmly bonding thermoplastic coating tape requiring an adhesive to pipes and other conduits.
One disadvantage of thermoplastic materials for use in coating for corrosion protection is that they tend to flow or sag due to gravity when they are hot. Since they are usually hot when applied to an article to be coated, the protective coating layer may be uneven and provide insufficient protection at some points on the surface of the coated article. The use of the wet fibrous material in the present invention eliminates these problems.
Thermoplastic tapes have been used by the pipeline industry as pipe coatings for more than 25 years. Such tapes are made as plastic film only and as film with an adhesive. Historically, these tapes have not required heating during application but with the advent of improved adhesives with high melting points, it became necessary to use heat during the application of many such tapes to pipes.
One of the drawbacks of using these new thermoplastic adhesives to bond thermoplastic coating tapes to metal pipes or other articles of manufacture is that their use usually limits the choice of thermoplastic coating materials to those capable of withstanding the temperatures necessary to develop the adhesive bond. In some cases, it may not even be necessary that the thermoplastic coating material melt or degrade for it to be impossible to apply it uniformly at high temperatures. At temperatures above 200.degree. F., with polyolefin tape especially, there is a reduction in its tensile and/or shear strength which may destroy its intended shape or structure during the coating process or interfere with the close contact between the thermoplastic adhesive and the metal pipe needed for good bonding by causing the tape to sag under the force of gravity.
There are existing coating tapes which do not exhibit this tendency to sag at the temperatures necessary to develop the adhesive bond. These generally utilize a mechanically prestressed cross-linked polyolefin backing which contracts when it is heated. The contraction counteracts the force of gravity as the tape is heated and compresses it onto the pipe. One disadvantage of this method is that prestressed cross-linked polyolefin coating tapes are relatively costly. Tape applied to a rotating pipe, as for example in the case of a mill application, will not sag but instead the tape will be nonuniformly affixed to the pipe. The nonuniformity can be ameliorated by precise control of wrapping tension and temperature but such procedures are very costly and limited to high capital investment and operating cost facilities.
The wet fibrous material performs the same function as the cross-linked polyolefin backing material discussed above, i.e., as it dries under the application of heat as discussed in more detail below, it contracts and compresses the tape or other coating material onto the pipe or other article of manufacture and counteracts the force of gravity. Thus, thermoplastics which tend to sag at the temperatures necessary to develop the adhesive bond may be used since the contraction of the fibrous material prevents sagging.
U.S. Pat. No. 3,190,780 by McNulty, et al., issued June 22, 1965, describes a method of applying a protective wrapping to pipe whereby an enamel is put on the pipe and then a thermoplastic film is put on the pipe over the enamel. An outer layer, which may be comprised of heavy kraft paper, is wrapped around the polyethylene film to prevent it from going "slack". U.S. Pat. No. 3,525,656 by Kennedy, issued Aug. 25, 1970, describes a method of simultaneously applying plastic film and a flexible carrier board to a pipe. The flexible board can be made of "industrial paper" and its purpose is to keep the plastic film from parting when it softens and it also protects the pipe and coating underneath. U.S. Pat. No. 3,245,856 by Morain, issued Apr. 12, 1966, describes a method of applying a coating to a pipe and immediately thereafter covering with a wrapping material which can be heavy felt or kraft paper. None of the above patents disclose the use of a wet fibrous material and the "paper" disclosed is of a heavy grade which is unsuitable for use in the present invention.
It is an object of the present invention to provide a method whereby thermoplastic coating materials and coating tapes can be firmly and evenly bonded to pipes and other articles of manufacture.
It is a further object of this invention to provide a coating method which overcomes the tendency of thermoplastic coating materials and coating tapes to deform under the application of heat.
It is still another object of the present invention to provide a method by which the overlapped areas of a spiral-wrapped thermoplastic coating tape can be compressed to provide an even coating surface.